Pseudomonas exotoxin and genetically modified forms of PE have been attached to monoclonal antibodies (mAbs) or growth factors to create cell specific cytotoxic agents. A mutant form of PE, LysPE38, has been chemically attached to mAb B3. This immunotoxin, B3-LysPE38 causes regression of human tumors in mice with a large therapeutic window. Clinical grade mAb B3 has been prepared and clinical grade LysP38 is being prepared for an immunotoxin trial scheduled to begin in 1992. Other mutant forms of LysPE38 have been evaluated for the production of immunotoxins, and one in which the lysine residues at the end domain III have been mutated makes a more active immunotoxin. A single chain immunotoxin (B3(Fv)-P38 KDEL) has been prepared and also causes tumor regression in mice. The conditions of producing B3(Fv)-P38 KDEL have been improved by studying refolding conditions and using mutant linker and connector molecules. Refolding can also be stimulated by the enzymes GroE and protein disulfide isomerase. The side chains of the amino acids in domain II have been changed to alanine residues to determine which side chains are important for the cytotoxic action of PE. Single domain immunotoxins have also been made with B3(Fv)-P38 KDEL and the results show that both the light and heavy chain interact with the B3 antigen. Indium labeled B3 has been prepared and shown to image tumors very well in nude mice. Single chain recombinant immunotoxins have been made with antibodies that react with the erbB2 oncogene, with an antibody C242 that binds with many human colon cancers and with antiTac that binds to the IL2 receptor present in many lymphomas and leukemias. These agents are in preclinical development. Other immunotoxins have been prepared that react with B cell lymphomas and the p75 subunit of the IL2 receptor. Recombinant toxins with longer half lifes have been created by inserting the CH2 domain of human IgG1 between the ligand and the toxin domains.